1. Technical Field
The present invention generally relates to methods and apparatus for processing received Orthogonal Frequency Division Multiplexing (OFDM) signals, and particularly relates to compensating for IQ imbalances in such signal processing.
2. Background
A typical wireless communications receiver includes a downconverter stage to translate the radio frequency carrier signal to an intermediate frequency (IF) signal and/or to baseband. Some receivers utilize direct down-conversion to a baseband signal (a so-called zero-IF signal). Many OFDM-based receivers in particular are expected to use direct-conversion zero-IF receivers.
Modern receivers typically implement complex down-conversion, whereby an input radio frequency signal is split and multiplied by a local oscillator and a quadrature version of the local oscillator. In effect, in one branch of the receiver the input signal is multiplied by a cosine waveform at the local oscillator frequency, while in the other branch the input signal is multiplied by a sine waveform at the local oscillator frequency. The resulting outputs are known as the in-phase (I) and quadrature (Q) components.
Imperfections in the analog components of the downconverter can result in imbalances between the two branches, with respect to the relative amplitudes, relative phases, or both. These IQ imbalances can in some cases have significant impact on the performance of a wireless receiver; the impact becoming more acute as higher-order modulation schemes are employed. This is particularly true in the direct-conversion zero-IF receivers contemplated for many future OFDM applications.
Previous attempts to correct for these imbalances have relied on adaptive statistical estimation techniques, wherein a reference signal is constructed, using estimates of the effects of the IQ imbalance, and compared to the actual received signal to generate an error signal. The error signal is adaptively minimized to refine the estimates of the statistics relating to the IQ imbalance. See, for example, U.S. Patent Application Publication US 2006/0029150. This approach is computationally complex, however, and may not be robust enough for all situations. For example, convergence rates may be unacceptably slow.